396 



EXPLORATION GEOPHYSICS 



projects an image of a portion of the azimuth compass card onto the film, thereby 

 incorporating this data onto the same film-frame with the rod image. 



The rod is of the fixed-target type, as shown in Figure 237. The fixed target 

 interval serves to calibrate the scale of the resulting picture at the particular distance 

 the rod is held from the instrument, thereby permitting vertical scaling of the film from 

 the level reference line to the datum target in terms of feet of elevation difference per 

 millimeter of film interval. The target material is chosen for certain desirable photo- 

 graphic properties, and yields good results regardless of field lighting conditions. Each 

 rod has three targets, any two of which must be visible in the picture in order to deter- 

 mine distance, except on very short shots, in which case any one of the targets will 

 give usable results. 



Instrumentation for Reading Data. — All readings and measurements are made 

 at the field office directly from the film negatives, using a reading device known as the 

 conversion projector, shown in Figure 239. The film is loaded into the projector proper, 

 which is located just below the screen and inside the main instrument case. The image 

 is projected onto the back surface of the screen by means of double reflections from 

 two front-surfaced mirrors. 



Fig. 239. — Conversion projector for measuring ele- 

 vation and distance values. (Courtesy of Republic 

 Exploration Company.) 



The projector and the low«r mirror are moved horizontally as a unit on a track by 

 means of the small crank located on the lower right side of the case. Reference lines 

 are drawn on the screen ; the top line is the index or zero line. The remainder of the 

 lines are used only to read the target intervals directly for short shots from about 20 to 

 400 feet distance. For longer distances, the readings are taken from a horizontal 

 graduated dial contained in the housing to which the small crank is attached. In opera- 

 tion, the frame under study is positioned in the projector with the image of the top 

 target on or very near the screen index line. The screen itself is then moved up or 

 down slightly by means of the zero adjusting lever until the screen index line centers 



